This invention relates to a liquid crystal display device and its driving method, and, more particularly, to constitution of a birefringence color liquid crystal display device and a driving method for a liquid crystal cell thereof.
Conventionally, liquid crystal display devices typically use a reflection-type liquid crystal display device which displays in monochrome employing a TN (twisted nematic) liquid crystal cell or an STN (super twisted nematic) liquid crystal cell. Generally, a transflective reflector is utilized as a reflector of a liquid crystal display device, and a backlight unit, such as an electro-luminescent (EL) light and a light emitting diode (LED) array, is provided outside the transflective reflector for visibility of the time display at night.
Recently, watches equipped with liquid crystal display device, portable tape recorders, cellular phones, portable game machines or the like are coming into fashonable, so a liquid crystal display device capable of colorful displaying is desired for them. Then, for example, a digital timepiece capable of color displaying by using a single-color liquid crystal display device which indicates white letters or the like on a blue or red background through a color polarizing film dyed with a dichroic pigment, has been developed.
However, for developing a timepiece that is more fashionable in design and portable machines that have stronger impact in appearance, it is not enough to use a single-color display device. Then, it is desired to provide a multi-color display device capable of displaying a plurality of colors.
It is proposed to mount a birefringence color liquid crystal display device in a timepiece or other portable machines to perform a multicolor display with the birefringence effect of liquid crystal by changing the voltage applied to a liquid crystal cell instead of using a color filter.
In order to change colors on a letter (in case of a watch, numerals to display normal time, an alarm time and a calendar) display portion using the birefringence color liquid crystal display device, RMS voltage of the signal supplied to the letter display portion must be variable. In order to change the effective value, an IC for driving liquid crystal that is capable of controlling gray scale is required, this results in an increase of development cost and an extension of the time period for development. Moreover, the complexity of driving circuits increases the size of the driver IC and the amount of current consumed.
As regards a birefringence color liquid crystal display device displaying in a multi-color, it is an object of the present invention to provide a way to display in a multi-color easily in which the birefringence color liquid crystal display cell is driven by a typical monochrome liquid crystal driving IC without a gray scale function for simple multi-color display at a low cost and low power consumption.
To attain the aforementioned object, the present invention provides a configuration for a liquid crystal display device, consisting of: a liquid crystal cell in which nematic liquid crystal is sandwiched and filled in a gap between a transparent first substrate, having first electrodes, and a transparent second substrate, having second electrodes; a pair of polarizing films respectively arranged on and under the liquid crystal cell; and a reflector arranged on a face of one of the polarizing films which is on the opposite side to the liquid crystal cell.
The display portion made up of the liquid crystal display cell consists of a letter display portion displaying in a single color and a mark display portion displaying in a plurality of colors.
A liquid crystal cell driving circuit for driving the liquid crystal display device to supply scanning signals to the first electrodes for the letter display portion, data signals to the first electrodes for the mark display portion, and data signals to the second electrodes for both the letter display portion and the mark display portion, is provided.
The reflector of the liquid crystal display device may be a transflective reflector. And a backlight unit for lighting the liquid crystal elements through the transflective reflector may be preferably provided on the opposite side of the reflector to the liquid crystal cell.
A retardation film or a twisted retardation film may be provided between the liquid crystal cell and the polarizing film positioned on the visible side thereof in the liquid crystal display device.
The liquid crystal cell is preferably an STN liquid crystal cell in which the nematic liquid crystal is aligned at a twist angle in the range from 180xc2x0 to 270xc2x0. Accordingly, a xcex94nd value which is the product of a value xcex94n in the birefringence of the liquid crystal and a gap d of the liquid crystal cell, preferably ranges from 1300 nm to 1600 nm.
In case of the above-mentioned liquid crystal display device having the retardation film, the liquid crystal cell is, preferably, an STN liquid crystal cell in which the nematic liquid crystal is aligned at a twist angle in the range from 180xc2x0 to 270xc2x0. Accordingly, a xcex94nd value which is the product of a value xcex94n in the birefringence of the liquid crystal and a gap d of the liquid crystal cell, preferably ranges from 1500 nm to 1800 nm, and a retardation value of the retardation film desirably ranges from 1600 nm to 1900 nm.
It is advisable that the retardation film forms relations of nx greater than nz greater than ny, where nx is the refractive index of the direction of a phase delay axis, ny is the refractive index in a direction orthogonal to the phase delay axis, and nz is the refractive index in a thickness direction.
In the use of the liquid crystal display device mentioned above having the twisted retardation film, the liquid crystal cell is, preferably, an STN liquid crystal cell in which the nematic liquid crystal is aligned at a twist angle in the range from 180xc2x0 to 270xc2x0. Accordingly, a xcex94nd value which is the product of a value xcex94n in the birefringence of the liquid crystal and a gap d of the liquid crystal cell, preferably ranges from 1500 nm to 1800 nm. A xcex94nd value of the twisted retardation film preferably ranges from 1400 nm to 1800 nm.
Another liquid crystal display device according to the present invention has: a first liquid crystal display device consisting of a first liquid crystal cell in which nematic liquid crystal is sandwiched and filled in a gap between a transparent first substrate having first electrodes and a transparent second substrate having second electrodes, a pair of polarizing films respectively arranged on and under the first liquid crystal cell, and a reflector arranged on a face of one of the polarizing films which is on the opposite side to the liquid crystal cell;
a second liquid crystal display device, arranged on a face of the first liquid crystal display device on the visible side, consisting of a second liquid crystal cell in which nematic liquid crystal is sandwiched and filled in a gap between a transparent first substrate having first electrodes and a transparent second substrate having second electrodes, and a third polarizing film arranged on a face of the second liquid crystal cell on the visible side; and
a liquid crystal cell driving circuit for driving the first and second liquid crystal display devices to supply scanning signals to the first electrodes of the first liquid crystal cell, data signals to the second electrodes of the first liquid crystal cell, and data signals to the first electrodes and the second electrodes of the second crystal liquid cell.
It is advisable that the second liquid crystal display device has a reflection-type polarizing film on the opposite side of the second liquid crystal cell from the visible side.
A driving method of the liquid crystal display device according to the present invention is for a birefringence color liquid crystal display device, as described hereinbefore, which consists of: a liquid crystal cell in which nematic liquid crystal is sandwiched and filled in a gap between a transparent first substrate having first electrodes and a transparent second substrate having second electrodes; a pair of polarizing films respectively arranged on and under the liquid crystal cell; and a reflector arranged on a face of one of the polarizing films, the face being on the opposite side to the liquid crystal cell, and in which a display portion thereof has a letter display portion displaying in a single color and a mark display portion displaying in a plurality of colors.
According to the method, the aforementioned liquid crystal cell is driven by supplying scanning signals to the first electrodes for the letter display portion; supplying data signals to the first electrodes for the mark display portion; and supplying data signals to the second electrodes for both the letter display portion and the mark display portion.
The present invention also provides a method for driving a liquid crystal display device which comprises: a first liquid crystal display device consisting of a first liquid crystal cell in which nematic liquid crystal is sandwiched and filled in a gap between a transparent first substrate having first electrodes and a transparent second substrate having second electrodes, a pair of polarizing films respectively arranged on and under the first liquid crystal cell, and a reflector arranged on a face of one of the polarizing films, the face being on the opposite side to the liquid crystal cell; and a second liquid crystal display device arranged on the visible side of the first liquid crystal display device and consisting of a second liquid crystal cell in which nematic liquid crystal is sandwiched and filled in a gap between a transparent first substrate having a first electrode and a transparent second substrate having a second electrode, and a third polarizing film arranged on a face of the second liquid crystal cell on the visible side. According to the method, the first and second liquid crystal cells are driven by supplying scanning signals to the first electrodes of the first liquid crystal cell while supplying data signals to the second electrodes thereof; and supplying data signals to the first electrode and the second electrode of the second liquid crystal cell.